Aromatic ester recovery

ABSTRACT

In the recovery of aromatic esters, a product of improved color is obtained when heated in the presence of antimony trioxide.

United States Patent Shmidl et al. 1 Feb. 22, 1972 [54] AROMATIC ESTER RECOVERY OTHER PUBLICATIONS [72] Inventors: Albert J. Shmidl, Crosby; Edgar C. Homoia,Ascited in Chem. Abstracts, 68, 2l7l0a(l968).

Winegartner, Baytown, both of Tex. [73] Assignee: Esso Research and Engineering Company Primary Weinberg Assistant Examiner-45. Jane Skelly [22] Filed: June 12, 1969 Attorney-Thomas B. McCulloch, Melvin F. Fincke, John S.

[211 App! 832,843 Schneider, Sylvester W. Brock, Jr., Kurt S. Myers and Timothy L. Burgess [52] U.S.Cl ..203/29, 203/50, 203/88, 7

260/475 B [51] Int. Cl ..C07c 67/06 5 ABSTRACT [58] Field of Search ..260/475 R, 47 S; 203/29, 50,

203/33 In the recovery of aromatic esters, a product of improved color is obtained when heated in the presence of antimony [56] References Cited trioxide.

UNITED STATES PATENTS 3,022,333 2/ 1962 Kalfadelis et al. ..260/47 S 4 Claims, No Drawings AROMATIC ESTER RECOVERY BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to the recovery of aro- 5 matic esters, especially dimethylterephthalate by heating in the presence of antimony trioxide to obtain a product of improved color.

2. Description of the Prior Art us. Pat. No. 2,879,289 us. Pat. No. 3,047,612

US. Pat. No. 3,047,613

us. Pat. No. 3,047,614

us. Pat. No.3,l23,633 I SUMMARY OF THE INVENTION DESCRIPTION OF THE PREFERRED EMBODIMENTS Aromatic esters are made in a number of differentmanners involving'the oxidation and esterification of aromatic feedstocks. In a preferred method, the aromatic feedstock containing at least one aromatic hydrocarbon ring substituted with a plurality of C, to C alkyl groups may be oxidized with molecular oxygen in the presence of an esterification compound such as methanol and catalyzed with a calytic amount of a polyvalent metal oxidation catalyst.

The various aromatic feedstocks which may be oxidized to produce the esters as well as the reaction conditions employed are known. Suitable reaction conditions are set forth, for example,'in U.S. Pat. Nos. 3,047,612; 3,047,613; 3,047,614 and 3,123,633. The product which is obtained in the esterfication process set forth in these patents is a crude product of aromatic ester, aromatic acid and other byproducts. Specifically disclosedin these patents is the oxidation and esterification of paraxylene to produce a product containing dimethylterephthalate.

To recover the organic ester, specifically dimethylterephthalatein highpurity, it is necessary to heat the crude product'such as in a distillation or flashing vesselto obtain the crude aromatic ester. This crude product is then'preferably recoveredby recrystallizing and'again heating such as .in a distillation column to obtain the aromatic ester in high purity. Other specific recoverymethods may be employed toobtain the aromatic ester in high purity.

According to the present invention, the aromatic esters are heated inthe presence of antimony trioxide. The presence of antimony trioxide in the heating steps produces a product of high purity without undesirable color. Further, problems of corrosion to steel and stainless steels are prevented. Furthermore, it has been found that the product heated in the presence of the antimony trioxide may also have a lower acid number than when the product is heated without the antimony trioxide being present. The amount of antimony trioxide used may be between 0.01 and L0 percent by weight. A preferred range is between 0.025 and 0.10 percent by weight.

In addition to the presence of antimony trioxide, the recovery and purification of aromatic esters may be carried out in the presence of a neutralization agent such as magnesium oxide and the like. Since the aromatic ester is often present with the corresponding acid, a neutralization agent will help in keeping the acid number low asthe aromatic ester is heated in the recovery method.

The production of aromatic esters such as dimethylterephthalate of high purity and having no undesirable color is of particular importance since the esters are used as monomers in the production of polyesters. A colored aromati TABLE I.-SIMPLE DIS'IILLATION 0F HERCULES DMT I Number 537 568 549 mu nn-" nn""nun-"um" 0.2gramsMg0 2.0grams CaGOa bhours 1.15 1.45 2% 2 hours-10 minutes 0. 89

Preheat time 3 Charge grams 3 Adde Distillation time i Percent bottoms--- irFraction we ,Acidit Color,

ight percent w, -l z----- percent transmission at420m hours at350 F.: 0- 1.- 2

l Activity: 0.4 meqJkg. (milliequivalents/kilogram).

ester or one having a high-acid number indicating the presence of an impure product is objectionable for use in making the polyester polymers.

The present invention will be further described and illustrated in the following examples which are set forth as illustrative of the invention and not as limitations on the scope of the invention.

problem of objectionable and undesirable color formation is I illustrated by distilling a high-purity dimethylterephthalate and measuring its color by the percentage transmission at 420p. at various time periods at 350 F. The results of the sim ple distillations in laboratory glass equipment are set forth in Table I hereinafter.

As is shown in Table l, merely heating a pure aromatic ester creates a color problem as is evidenced by the decrease from -late and then obtaining a heart out from a final distillation.

The significance of having the antimony trioxide present while heating the aromatic ester during distillation is shown in Table ll hereinafter.

TABLE IL-PURIFIGATION OF CRUDE FLAKED DM'I Simple Distillation distillation recrystalrecrystallization simple iization simple Processing scheme distillation distillation Initial distillation:

Number of plates i0 0 Reflux ratio 2/1 0 Pressure, mm. of Hg 760 760 Absolute:

Additive, SbrOi, p.p.m 1000 0 Overhead fraction, weight percent. 83 96 Bottoms fraction, weight percent-.. 2 4 Loss, weight prcent 15 Recrystallization in paraxylene:

Yield, weightpercent. 81 90 Acidity, m q-lkg 13 2+ Final distillation:

Number of plates-.. 0 0 Reflux ratio o 0 i Pressure mm. of Hg 760 760 Absolute:

Additive, SbiOa, ppm 1000 0 Front end iraction, weight percent 17 26 Percent transmission at 420 mu after 0 and 6 hours at 350 F /100 98/96 Acidity, meqJkg 5. 4 20+ Heart cut fraction, weight percent 68 42 Percent transmission at 420 m after 0 and 6 hours at 350 F 100/100 81/75 Acidity, meqJkg 1. 7 20+ Bottoms, weight percent. 13 8 Loss, weight percent 2 24 a 600 p.p.m. of

timony trioxide in both the initial distillation and final distillation resulted in a heart cut fraction having no undesirable color. Further, it is noted that the acidity of the recovered dimethylterephthalate is considerably lower than that when no antimony trioxide was present.

The nature and object of the present invention having been completely described and illustrated and the best mode thereof contemplated set forth, what we wish to claim as new and useful and secure by Letters Patent is:

1. In a method for recovering dimethylterephthalate, produced by direct oxidation-esterification of paraxylene in ,the presence of methanol, free oxygen containing gas, and a 'polyvalent metal oxidation catalyst to produce a product containing dimethylterephthalate, the step of recovering said dimethylterephthalate by heating said product to distillation or flashing temperature in the presence of an amount of antimony trioxide between 0.01 and LO percent by weight.

2. A method in accordance with claim 1 wherein said heating is in the flashing of said product to recover purified dimethylterephthalate.

3. A method in accordance with claim 1 wherein said heating is in the distillation of said product to recover purified dimethylterephthalate.

4. A method according to claim 1 wherein said antimony trioxide is present in the amount of between 0.025 and 0.1

0 40 percent by weight. 

2. A method in accordance with claim 1 wherein said heating is in the flashing of said product to recover purified dimethylterephthalate.
 3. A method in accordance with claim 1 wherein said heating is in the distillation of said product to recover purified dimethylterephthalate.
 4. A method according to claim 1 wherein said antimony trioxide is present in the amount of between 0.025 and 0.1 percent by weight. 